Emulator - The Easiest to Try
This is the least appealing option from an audio perspective. The emulators don't really try to stay true to the original Commodore PET free running shift-register based square wave sound.
But it works, if you want to mess with it - Great for development!
Download the file, and run your emulator. In VICE you can auto-boot and load the .PRG to have it start immediately. Start playing the keyboard and have fun.
For an Emulator, I use VICE:
There's PC and Mac versions, as well as other ports.
Tape Drive (Datassette)
Requires a PET and Tape Drive, as well as a Blank Tape
Updated: I've found a way to get the file small enough to distribute myself! The file is recorded in 8 bit with a sample rate of 8 kHz and has been tested as working on a real tape with a real PET.
Download the program file in .FLAC format
To use PetSynth on a PET with a tape drive or datassette, you first need to download the .flac version of the program binary. This is an audio file recorded from an actual PET onto tape, and not from a conversion program.
Once you have the file, you need a program that can play .flac, or can convert it to whatever format you wish, as long as it's a lossless format, like aiff or wav, but not a lossy format like .mp3 or .ogg, as these will changes the sound in ways which the human ear can't really hear, but the computer can.
Recording to and Audio Tape
Next, connect the output of your computer's sound card to a tape recorder of some kind. Press record on the tape recorder and hit play on your computer.
Make sure you get the audio tape recording as loud as possible without being too loud. If you have problems loading the program, trying re-recording and turning the volume up or down depending on what the problem is. It's tricky.
When the playback is done, stop the tape. You're ready to load it on a real PET.
Then start the PET and type LOAD and press enter. The PET will say "PRESS PLAY ON TAPE"
Then insert the tape into PET tape unit and press play on the tape drive.
When it stops, type "RUN" and hit enter.
Play PetSynth and rock on!
PLEASE NOTE: The PET, once PetSynth or any other sound using program has been running, may leave the shift register used to generate the sound in a weird state. It's fine if you want to run PetSynth again, but if you want to record (or perhaps load, I'm not sure) with the tape, you may need to restart your PET.
Converting from the .PRG file.
If you'd rather download the .PRG file, instead of the .FLAC file, you may have success converting it yourself.
Download the .PRG file. You're going to need a program to convert it to a .WAV using software you can find here:
http://www.zimmers.net/anonftp/pub/cbm/crossplatform/transfer/datassette/index.html
I don't have first hand experience with the process, but I'll write up a quick guide as soon as I can properly test a working process.
Disk Drive
First, you need a disk drive for the Commodore PET. There's a few. Then you need a way to plug it into your PC. I don't do that, so I don't know how that would work.
However, if you get a Commodore 1541 disk drive (used with the C64 and VIC-20) and a XE or other cable, you can use the PC's parallel port to connect to it and transfer the file to a disk formatted by a PET in the drive. Then you can use that disk in the PET's disk drive.
For a cable, I use a XM1541:
http://sta.c64.org/xm1541.html
For a transfer program, I use CBMXfer:
http://www.6502.org/users/sjgray/software/cbmxfer/cbmxfer.html
Make sure you set your PC's parallel port in whatever mode the software above needs. For example, sometimes you need to put the parallel port into "SPP" mode. You may also need to set the speed or change other setting in your transfer program. Follow the instruction on the pages linked to above for help.
Buying Pre-Made Tapes and Disks from Me
If you want to buy a pre-made disk from me, let me know by dropping me an email at: chiron (at) chiron bramberger (dot) com
If enough people are interested I may have the resources to whip up a batch if I can cover my costs. Thanks!